: The hypothesis to be tested is that during pregnancy the relationship between RIHP and sodium excretion becomes gradually attenuated due to a gradual increase in the compliance of the renal interstitium. The combination of increased renal interstitial compliance and reduced natriuretic sensitivity to increases in RIHP leads to an attenuated increase in RIHP and less sodium excretion and the gradual volume retention and progressive volume expansion during the course of pregnancy. The proposed studies will determine the relationship between renal perfusion pressure (RPP), RIHP, and sodium excretion in pregnancy. Also, the compliance of the renal interstitium and the sensitivity of the natriuretic response will be determined with selective increases in RHIP by direct renal interstitial volume expansion (DRIVE) utilizing a chronically implanted polyethylene matrix in the left kidney.